(1) Field of the Invention
The present invention relates to an audio decoding apparatus which decodes a compressed audio stream, and particularly to an audio decoding apparatus which simultaneously decodes a plurality of audio streams and outputs the decoded audio streams as audio signals.
(2) Description of the Related Art
With the advancement of communication technology and the diversification of audio/video devices in recent years, various types of decoder LSI which decode compressed video and audio signals are being provided.
For example, a high-performance LSI which simultaneously decodes the high-vision video signals for two channels is provided (see non-patent document 1: NEC Electronics, “System LSI μPD61160 for Digital Hi-Vision Television”, Internet <URL:http:// www.necel.com/digital_av/japanese/mpegdec/d61160.html>, Aug. 30, 2004 search). With this LSI, it is possible to view the video of one television program and simultaneously record the video of another television program.
However, although the aforementioned conventional LSI includes two video decoders, with respect to audio decoders, only one is included. As such, with regard to video, streams for two channels can be decoded simultaneously. However, with respect to audio, there is no guarantee that streams for two channels can be decoded simultaneously at all times. In other words, depending on the content and timing of audio streams received, situations arise in which simultaneous decoding is not possible, and as such, problems such as sound interruption and noise, for example, can occur.
Here, it is possible to presume that problems such as sound interruption, and the like, can easily be avoided by adopting a structure in which two audio decoders are included in one LSI. However, in performing a transmission control in which two output processing units are simply made to output Pulse Code Modulation (PCM) data from two decoders on a per frame basis to implement the simultaneous decoding of audio streams of two completely independent channels, sound interruption, and the like, may still occur as there are cases where the number of samples per frame are different for the two audio streams.
FIG. 1 is a timing chart explaining the mechanism for the occurrence of sound interruption that may arise in the case where two audio streams having a different number of samples per frame, are decoded using two audio decoders. ADEC1 and ADEC2 are first and second audio decoders, respectively, which decode separate compressed audio streams and output PCM data. AOUT1 and AOUT2 are first and second output processing units, respectively, which perform digital/analog (D/A) conversion, and so on, on the PCM data and output the result as audio signals. APCM is an output control unit which transmits, on a per frame basis, the respective PCM data outputted from ADEC1 and ADEC2, to AOUT1 and AOUT2, respectively. In the diagram, the vertical axes indicate the processing time and processing order of the frame-units of data, with the passage of time shown moving in the downward direction. Rectangular blocks 1a to 1f, and 2a to 2f, in the temporal axis direction, respectively correspond to one frame of data (the same marking being given to the same frame).
As shown in FIG. 1, sound interruption occurs between the outputting of audio for frame 2c and 2d by AOUT2. This is because, although under normal circumstances, the APCM should receive frame 2d outputted by ADEC2 and transmit this to AOUT2 (output request 2 should be given) immediately upon receiving an output completion notification 2 from AOUT2 which has finished the audio output of frame 2b, frame 2d could not be transmitted to AOUT2 as the decoding by ADEC2 is not yet completed (frame 2d still being decoded). Moreover, the reason why the APCM is unable to standby for the completion of the decoding of frame 2d by ADEC2 is because PCM data for MIX sound, such as earthquake warnings and the sound effects emitted during remote control operation, which require real-time performance, need to be outputted to AOUT2, together with the audio frame, in a regular cycle.
The reason why the APCM is unable to output the frame 2d to AOUT2 in the usual (normal) timing can be traced back to the late timing at which the APCM issues a decoding request (DEC request 2) to ADEC2, and further traced back to the excessively long time spent by the APCM in the transmission of frame 1c from ADEC1 to AOUT1. This is because, as the number of samples per frame of the audio stream inputted to ADEC1 is greater than that of the audio stream inputted to ADEC2, the transmission from ADEC1 of frame data having the greater number of samples, requires a longer time, and the processing with respect to ADEC2 (decoding request, and so on) is delayed.